98 E. D. Sedlin, L. Sonnerup 



Rheologlcal Considerations in the Physical Properties of Bone "" 



E. D. Sedlin '''•", L. Sonnerup '•'''"'•' 



The Biomechanics Laboratory, Department of Orthopaedic Surgery, University of Goteborg, 



Goteborg, Sweden 



Our studies of the physical properties (p.p.) of bone have concerned three general 

 questions. (1). How are the p.p. altered by changing the method of testing, storage, 

 and preparation? (2) How may one account for the variations that are noted? (3) How 

 is bone to be classified in the general realm of physical bodies insofar as its mechanical 

 behavior? A complete description of all of our findings and methods is beyond the 

 scope of this presentation, and is presented elsewhere (Sedlin and Hirsch, in press). 

 Some of the findings relative to questions 1 and 2 are noted below. These are based 

 upon compression, bending, and tension tests on more than 500 samples of femoral 

 cortical bone obtained from 20 autopsy subjects. The tests were performed wet, 

 usually at body temperature, on an Instron Tensile Test Machine. 



a. There is no significant effect of temperature on maximum stress (om) and energy 

 absorption, in the range from 21 ~C to 37 C. The modulus of elasticity (E) may 

 increase slightly at 21 ^'C. 



b. Air drying alters the p.p. in less than 1 hour in most cases. 



c. Freezing of specimens prior to testing increases on slightly, but does not change 

 any other p.p. 



d. Fixation in formalin does not appear to alter E. 



e. In small sample testing, size of specimen does not contibute to the variation 

 noted in ranges tested. (1 mm X 2 mm to 3 mm X 3 mm) 



f. Repeated loading of the same specimen does not change E, if loads of the order 

 of 35''/o of the failure load are used, and one minute or more be allowed for recovery 

 between loads. 



g. In bending tests, E can be significantly changed by changing the distance 

 between supports. 



h. It is important to measure specimen size before testing. 



i. There are significant differences in om and E in specimens from different quad- 

 rants of the mid-femur, the lateral ranking highest, with posterior lowest in regard 

 to these variates. 



The volume occupied by the Haversian canals in these specimens did not corre- 

 late well with the p.p. in the majority of instances. 



k. Significant differences exist in the p.p. of bone from different individuals that 

 are not accounted for by age. 



1. An high correlation exists between o^[ and E in bending. 



m. Testing of small samples appears to be a reliable method for the demonstration 

 of differences in p.p. between individuals. 



During conduct of the experiments indicated above, some information became 

 available that provided insight into the third question posed. Some background is 

 necessary before considering this question. 



* Work supported in part by U.S. Nation.iI Institutes of Health and Swedish Medical Research Council. 

 ** Special Research Fellow, U.S. National Institutes of Health. 

 *'"'■■ University Lecturer, Dept. of Strength of Materials, Chalmers Tekniska Hogskola, Goteborg. 



